The New York Times of June 26, 2017 contains an article entitled “Carbon in Atmosphere Is Rising, Even as Emissions Stabilize”, discussing the recent atmospheric CO2 growth rate. It contains statements such as “The excess carbon dioxide scorching the planet rose at the highest rate on record in 2015 and 2016. A slightly slower but still unusual rate of increase has continued into 2017.”

Is this really that exceptional? Let’s look at the facts:

This bar chart from NOAA’s Earth System Research Laboratory (NOAA-ESRL) displays the annual increase in atmospheric CO2 recorded at the Mauna Loa station on Hawaii. The growth rate in the last two years, 2015 and 2016 both exceed any increase observed in previous years. How does this fit to the recent finding by the Global Carbon Project (GCP) that the global anthropogenic CO2 emissions have almost levelled over the last few years?

NOAA-ESRL computes the annual CO2 growth rate by averaging the 4 monthly mean observations from November to February and then subtracting the average of the mean observations from the same months one year before. These are the values plotted in the bar chart shown above. If one uses the same recipe but calculates the increase not only over the calendar year, but for 12-month intervals lagged by 1 month, one obtains the following time series:

This graph includes also the CO2 growth rate observed at the South Pole station. (The monthly atmospheric observations were obtained from the Scripps CO2 Program). The two records are remarkably similar, indicating that measurements from either station reflect to a good approximation the change in the global inventory of atmospheric CO2. At both stations the CO2 growth rate peaked at the end of 2015, exceeding any previously recorded 12-month increase. This recent peak coincides with the El Nino from 2015/2016. Since the peak the growth rate shows now a decreasing trend similar to previous El Nino periods (e.g. in 1997/1998, 1988/1989, 1973/1974).

How would we expect the atmospheric CO2 growth rate to evolve given the reported anthropogenic CO2 emissions? The graph below shows in the upper panel the observed CO2 growth rate from Mauna Loa (green curve) together with predictions by two simple global carbon cycle models with prescribed anthropogenic emissions as shown in the lower panel (Technical details about the models and the data can be found at the bottom of this page). The grey background shading in the upper panel indicates El Nino conditions (characterised by the MEI index).

These simple models do not include any climate variability; they simply describe how CO2 emissions are redistributed among atmosphere, ocean and land biosphere, based on our basic understanding of carbon uptake and turnover in these three reservoirs.

Obviously, the year to year variability of the observed CO2 growth rate can not be captured by these simple models but is strongly correlated to the El Nino – Southern Oscillation, discovered already long ago (Bacastow, 1976, Nature). During El Nino phases, south-eastern Asia and also parts of the Amazon region are drier than normal, causing reduced photosynthesis but also fostering vegetation fires. Both processes lead to a temporary negative carbon balance of the terrestrial biosphere, which is mirrored as an anomalous increase in the atmospheric CO2 growth rate.

On the other hand, the simple global carbon cycle models reproduce quite faithfully the longer term trend including decadal variations of the atmospheric CO2 growth rate. This can be taken as an indication, that, at least under present conditions, the first order dynamics of the global carbon cycle are well represented in these models.

Why is atmospheric CO2 not yet reflecting the reported “stabilisation” of the anthropogenic emissions during the recent years?

The emissions from fossil fuel burning have indeed flattened over the last ~3 years (reported emissions are only available up to 2015; the grey point for 2016 in the emission graph above is an extrapolation to the year 2016 made by the GCP). On the other hand, emissions from changes in land use (a.o. deforestation) have picked up over the last few years so that the total anthropogenic emissions (brown curve) are still increasing with a similar trend as over the last 15 years. One has to realise, however, that the land use emissions for the years 2011-2015 are also only an estimate, made by the GCP based on scaled biomass burning emissions derived from satellite measurements and are thus quite uncertain.

A second effect to consider is the dynamics of the global carbon cycle. Keeping constant total emissions beyond 2015 (grey points), the simple carbon cycle models predict only a very slow reduction of the atmospheric CO2 growth rate (see the model curves in the upper panel). Given the large inter annual variations, it would take many years to detect such a small declining trend in the atmospheric CO2 growth rate. Of course, if the anthropogenic emissions were to be reduced from their present values, as required to meet e.g. the Paris agreement to limit climate change in this century, then a declining trend in the atmospheric CO2 growth rate should become larger and eventually easier to detect.

Thus, given the available information, the global carbon cycle behaves as expected: the recent El Nino left it’s imprint on the atmospheric CO2 growth rate, but it was not exceptional compared to previous El Nino periods. Furthermore, the longer-term increase of the growth rate is consistent with the reported anthropogenic emissions. But this does not mean that we do not have to worry: We are discussing here changes in the atmospheric CO2 growth rate, not the atmospheric CO2 concentration! As long as the growth rate is positive, atmospheric CO2 continues to increase. Stabilising emissions does not stabilise atmospheric CO2! – a fact that is long known in the carbon cycle science community, but often forgotten in the public debate.

Technical details: The two simple global carbon cycle models used to calculate the expected atmospheric CO2 growth rate are the “box-diffusion model” of Oeschger et al. (1975, Tellus), and the “Bern-SAR model” as used by IPCC in the second assessment report (1995) for the calculation of the global warming potentials of various climate forcing agents. The Bern-SAR model is linear in the impulse response variant used here (see Joos et al., 2013, ACP), the box-diffusion model includes the non-linear ocean carbonate chemistry. Neither of these models include any climate feedback effects and are thus of limited use beyond the present conditions. Fossil fuel and land use CO2 emissions are obtained from the GCP (Le Quéré et al., 2016, ESSD).